The invention relates generally to optical scanners, and in particular to scanners used for scanning two-dimensional optical codes such as bar code symbols.
Optical codes are patterns made up of image areas having different light reflective or light emissive properties, which are typically assembled in accordance with a priori rules. The term “bar code symbol” is sometimes used to describe certain kinds of optical codes. The optical properties and patterns of optical codes are selected to distinguish them in appearance from the background environments in which they are used. Devices for identifying or extracting data from optical codes are sometimes referred to as “optical code readers” of which bar code scanners are one type. Optical code readers are used in both fixed or portable installations in many diverse environments such as in stores for check-out services, in manufacturing locations for work flow and inventory control and in transport vehicles for tracking package handling. The optical code can be used as a rapid, generalized means of data entry, for example, by reading a target bar code from a printed listing of many bar codes. In some uses, the optical code reader is connected to a portable data processing device or a data collection and transmission device. Frequently, the optical code reader includes a handheld sensor that is manually directed at a target code.
Generally, it is necessary to align the scanning beam with the optical code in order to read the code. For example, with the common one-dimensional bar code symbol, which has a pattern of variable-width rectangular bars separated by fixed or variable width spaces (with the bars and spaces having different light reflecting characteristics), the scanner is typically aligned so that the scanning beam moves across all or at least a large fraction of the bars and spaces in one pass.
Some optical codes are two-dimensional, such as the well known PDF417 two-dimensional bar code symbol, in which a pattern of light and dark elements are arranged in rows. A description of the PDF417 bar code symbol and techniques for decoding it are disclosed in U.S. Pat. No. 5,635,697 to Shellhammer et al. and assigned to Symbol Technologies, Inc., which patent is incorporated herein by reference. Other two-dimensional optical codes are known (e.g., MaxiCode, described in the publication, “International Symbology Specification—Maxicode”, by AIM International, Inc.). Typically, a two dimensional optical code comprises a grid tiled by regular polygons such as squares or hexagons. Typically a black or white feature or polygon is located at each grid location.
Alignment of the scanning beam can be more important in the case of two-dimensional optical codes than with one-dimensional codes. For example, with the PDF417 symbol, the scanning beam typically moves in a raster pattern, in which the beam traces out a series of spaced apart rows that cover a rectangular area (much as the beam of a cathode ray tube moves across the top row, then the second row, and so forth until reaching the bottom row, before starting over again at the top row). It can be desirable for the rows of the raster pattern to be aligned with rows of the two-dimensional bar code.
Sometimes, particularly in the case of handheld bar code readers, it is desirable to give visual clues to the user to facilitate the desired alignment. For example, with a typical one-dimensional bar code reader, the user sees a visible line (typically a red line) corresponding to the path of the scanning beam, and the line is simply aligned with the bar code symbol.